U.S. patent application number 11/911702 was filed with the patent office on 2010-02-25 for parallel arranged linear amplifier and dc-dc converter.
This patent application is currently assigned to NXP B.V.. Invention is credited to Pieter G. Blanken, Brian Minnis, Paul Anthony Moore, Derk Reefman.
Application Number | 20100045247 11/911702 |
Document ID | / |
Family ID | 36658750 |
Filed Date | 2010-02-25 |
United States Patent
Application |
20100045247 |
Kind Code |
A1 |
Blanken; Pieter G. ; et
al. |
February 25, 2010 |
PARALLEL ARRANGED LINEAR AMPLIFIER AND DC-DC CONVERTER
Abstract
A power supply system comprises a parallel arrangement of a
linear amplifier (LA) and a DC-DC converter (CO). The linear
amplifier (LA) has an amplifier output to supply a first current
(II) to the load (LO). The DC-DC converter (CO) comprises: a
converter output for supplying a second current (12) to the load
(LO), a first inductor (L1), and a switch (SC) coupled to the first
inductor (L1) for generating a current in the first inductor (L1),
and a low-pass filter (FI) arranged between the first inductor (L1)
and the load (LO). The low pass filter (FI) comprises a first
capacitor (C1; CA) which has a first terminal coupled to the switch
(SC) an a second terminal coupled to a reference voltage level
(GND), and a second inductor (L2; LC) which has a first terminal
coupled to the first inductor (L1) and a second terminal coupled to
the load (LO). The low-pass filter further comprises, either: (i) a
series arrangement of a second capacitor (C2) and a damping
resistor (R2), which series arrangement is arranged in parallel
with the first capacitor (C1), or (ii) a parallel arrangement of a
third capacitor (CB) and a damping resistor (RB) arranged in series
with the first capacitor (CA), or (iii) a series arrangement of a
third inductor (L3) and a damping resistor (R3), which series
arrangement is arranged in parallel with the second inductor (L2),
or (iv) a parallel arrangement of a fourth inductor (LD) and a
damping resistor (RD), which parallel arrangement is arranged in
series with the second inductor (LC).
Inventors: |
Blanken; Pieter G.; (Nuenen,
NL) ; Moore; Paul Anthony; (Seaford, GB) ;
Reefman; Derk; (Best, NL) ; Minnis; Brian;
(Crawley, GB) |
Correspondence
Address: |
Docket Clerk
P.O. Box 802432
Dallas
TX
75380
US
|
Assignee: |
NXP B.V.
Eindhoven
NL
|
Family ID: |
36658750 |
Appl. No.: |
11/911702 |
Filed: |
April 12, 2006 |
PCT Filed: |
April 12, 2006 |
PCT NO: |
PCT/IB06/51136 |
371 Date: |
October 26, 2009 |
Current U.S.
Class: |
323/273 |
Current CPC
Class: |
H03F 3/217 20130101;
H03F 2200/432 20130101; H03F 3/211 20130101 |
Class at
Publication: |
323/273 |
International
Class: |
G05F 1/10 20060101
G05F001/10 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 20, 2005 |
EP |
05300298.6 |
Aug 29, 2005 |
EP |
05300700.1 |
Apr 12, 2006 |
IB |
PCT/IB2006/051136 |
Claims
1. A power supply system comprising a parallel arrangement of a
linear amplifier and a DC-DC converter, wherein: the linear
amplifier has an amplifier output for supplying a first current to
the load, and the DC-DC converter comprises a converter output for
supplying a second current to the load, a first inductor, and a
switch coupled to the first inductor for generating a varying
current in the first inductor, and a low-pass filter arranged
between the first inductor and the load, the low pass filter
comprises: a first capacitor having a first terminal coupled to the
switch and a second terminal coupled to a reference voltage level,
a second inductor having a first terminal coupled to the first
inductor and a second terminal coupled to the load, and either: a
series arrangement of a second capacitor and a damping resistor,
which series arrangement is arranged in parallel with the first
capacitor, or a parallel arrangement of a third capacitor and a
damping resistor, which parallel arrangement is arranged in series
with the first capacitor, or a series arrangement of a third
inductor and a damping resistor, which series arrangement is
arranged in parallel with the second inductor, or a parallel
arrangement of a fourth inductor and a damping resistor, which
parallel arrangement is arranged in series with the second
inductor.
2. A power supply system as claimed in claim 1, wherein, in use,
the second current provides a DC and low frequency portion of a
total current through the load, the first current provides a high
frequency portion of the total current through the loading, a
crossover frequency being defined as the frequency at which the
high frequency portion is equal in magnitude to the DC and low
frequency portion, and wherein a bandwidth of the low-pass filter
is selected above the crossover frequency.
3. A power supply system as claimed in claim 1, wherein a bandwidth
of the low-pass filter is selected below a switching frequency of
the DC-DC converter to obtain a current transfer suppression of the
low-pass filter at the switching frequency.
4. A power supply system as claimed in claim 1, wherein the low
pass filter comprises the second inductor and the series
arrangement of the second capacitor and the damping resistor, and
wherein the second capacitor has an impedance which is at least two
times smaller than the impedance of the first capacitor.
5. A power supply system as claimed in claim 4, wherein the first
capacitor, the second capacitor and the second inductor form a
resonance circuit having a first resonance frequency determined by
values of the first capacitor, the second capacitor and the second
inductor, and a second resonance frequency determined by the first
capacitor and the second inductor, the first resonance frequency
being lower than the second resonance frequency, and wherein values
of the first capacitor, the second capacitor and the second
inductor are selected to obtain the second resonance frequency
lower than a switching frequency of the DC-DC converter and higher
than a crossover frequency, wherein the crossover frequency is
defined as the frequency at which, in use, the first current which
provides a high frequency portion of a total current through the
load is equal in magnitude to the second current which provides a
DC and low frequency portion of the total current through the
load.
6. A power supply system as claimed in claim 1, wherein the low
pass filter comprises the second inductor, and the series
arrangement of the third inductor and the damping resistor, and
wherein the third inductor has an impedance which is at least two
times smaller than the impedance of the second inductor.
7. A power supply system as claimed in claim 6, wherein the first
capacitor, the second inductor, and the third inductor form a
resonance circuit having a first resonance frequency determined by
values of the first capacitor and the second inductor, and a second
resonance frequency determined by the first capacitor, the second
inductor, and the third inductor, the first resonance frequency
being lower than the second resonance frequency, and wherein values
of the first capacitor, the second inductor, and the third inductor
are selected to obtain the second resonance frequency lower than a
switching frequency of the DC-DC converter and higher than a
crossover frequency, wherein the crossover frequency is defined as
the frequency at which, in use, the first current which provides a
high frequency portion of a total current through the load is equal
in magnitude to the second current which provides a DC and low
frequency portion of the total current through the load.
8. A power supply system as claimed in claim 1, wherein the linear
amplifier comprises: a first amplifier stage having an output
directly connected to the load for supplying the first current to
the load, a second amplifier stage for generating a third current
being proportional to the first current, the first amplifier stage
and the second amplifier stage having matched components, and a
differential input stage having a non-inverting input for receiving
a reference signal, an inverting input for receiving a voltage
proportional to a system output voltage across the load, and an
output being coupled to both an input of the first amplifier stage
and an input of the second amplifier stage, and wherein the DC-DC
converter further comprises a controller having a control input for
receiving a voltage generated by the third current to control the
second current for minimizing a DC-component of the first
current.
9. An apparatus comprising the power supply system as claimed in
claim 1, wherein the load comprises a circuit of the apparatus.
10. An apparatus as claimed in claim 9, the apparatus comprising a
telecom system wherein the load comprises an RF amplifier.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a power supply system with a
parallel arrangement of a linear amplifier and a DC-DC converter,
and an apparatus comprising such a power supply system.
BACKGROUND OF THE INVENTION
[0002] U.S. Pat. No. 5,905,407 discloses a high efficiency power
amplifier using combined linear and switching techniques with a
feedback system. A linear amplifier supplies an output current to a
load via a sense resistor. A switching amplifier which comprises a
controllable switch and two series arranged LC-sections is used as
a DC-DC converter and supplies a further output current to the
load. The resistor is arranged between the output of the linear
amplifier and the output node of the power supply system at which
the output voltage is present across the load. The output current
of the linear amplifier flows through this resistor. The voltage
across the resistor is used to control the DC-DC converter to
obtain a minimal DC-component of the output current of the linear
amplifier. Preferably, this minimal DC component is zero.
[0003] This parallel arrangement of the linear amplifier and the
DC-DC converter is applied in a radio transmitter. The radio
transmitter comprises a power supply reference generator which
supplies a reference signal to the linear amplifier to generate the
system output voltage which tracks the reference signal. The radio
transmitter further comprises a radio frequency (further referred
to as RF) power amplifier for amplifying an RF signal. The RF
amplifier is coupled to the output node to receive the system
output voltage as a supply voltage. The reference signal is
modulated to follow an amplitude modulation of the input signal of
the RF amplifier. Thus, the supply voltage of the RF amplifier is
controlled to meet the needs of the RF power amplifier to improve
the efficiency of the RF amplifier.
[0004] The relatively slow DC-DC converter supplies the DC and low
frequent currents to the load at relatively high power efficiency,
and the relatively power inefficient linear amplifier supplies the
high frequent currents to the load only.
[0005] The switching amplifier comprises a two-stage LC-filter. The
two inductors of the LC-filter are arranged in series between the
load and a switch of the switching amplifier which switch is
connected to the DC input voltage. One of the capacitors of the
LC-filter is connected between the junction of the two inductors
and ground, the other capacitor of the LC-filter is connected in
parallel with the load. The voltage at the junction of the two
inductors is used by the feedback network to influence the control
of the switches of the switching amplifier.
SUMMARY OF THE INVENTION
[0006] It is an object of the invention to provide a parallel
arranged linear amplifier and DC-DC converter with a less complex
control of the DC-DC converter.
[0007] A first aspect of the invention provides a power supply
system with a parallel arrangement of a linear amplifier and a
DC-DC converter as claimed in claim 1. A second aspect of the
invention provides an apparatus comprising the power supply system
as claimed in claim 9. Advantageous embodiments are defined in the
dependent claims.
[0008] The power supply system comprises a parallel arrangement of
a linear amplifier and a DC-DC converter. The linear amplifier
supplies a first current to the load which contains the high
frequency components of the current drawn by the load. The DC-DC
converter (further also referred to as converter) has a converter
output to supply the second current to the load which contains the
DC and low frequency components of the current drawn by the load.
The converter further comprises a first inductor, and a controlled
switch coupled to the first inductor to generate a varying current
in the first inductor. The power supply system further comprises a
low-pass filter arranged between the first inductor and the load.
The low pass filter comprises: a first capacitor which has a first
terminal coupled to the switch and a second terminal coupled to a
reference voltage level, and a second inductor which has a first
terminal coupled to the first inductor and a second terminal
coupled to the load. The low pass filter further comprises one of
the following sub-circuits:
[0009] (i) a series arrangement of a second capacitor and a damping
resistor, which series arrangement is arranged in parallel with the
first capacitor, or
[0010] (ii) a parallel arrangement of a third capacitor and a
damping resistor, which parallel arrangement is arranged in series
with the first capacitor, or
[0011] (iii) a series arrangement of a third inductor and a damping
resistor, which series arrangement is arranged in parallel with the
second inductor, or
[0012] (iv) a parallel arrangement of a fourth inductor and a
damping resistor, which parallel arrangement is arranged in series
with the second inductor.
[0013] The common issue is that the damping resistor is arranged in
series with a capacitor or in parallel with an inductor. This in
contrast to the prior art converter applications, wherein only
additional LC filters are used without damping. However, these
relatively lossless additional LC filters have a high quality
factor and thus cause undesirable resonances. The prior art U.S.
Pat. No. 5,905,407 suppresses these resonances by sensing the
voltage at the input of the additional LC filter, and by adapting
the feedback. This complicates the feedback system and may lead to
instabilities or impaired performance of the feedback loop. It is
commonly known, in small signal filtering applications, to damp
resonances in LC filters with a damping resistor which is present
in the main current loop. However, in these small signal filters a
dissipation in the damping resistors is not an issue. In contrast,
in low-pass filters which filter the output current of a DC-DC
converter the power efficiency of the converter is a very relevant
issue. Implementing damped small signal filter topologies in a
filter for a DC-DC converter is not obvious because these have the
commonly accepted drawback that the power efficiency of the
converter is compromised by the high dissipation in the damping
resistor.
[0014] The invention provides a low-pass filter in a power supply
system which comprises a parallel arrangement of a linear amplifier
and a DC-DC converter, which filter has a special construction to
avoid additional DC power dissipation in the damping resistor,
while providing good HF suppression.
[0015] The invention is based on the insight that the damping
resistor should not be present in main current loop of the
converter. The damping resistor may be arranged in series with a
capacitor to a reference voltage which usually is ground. Or, the
damping resistor is arranged in parallel with an inductor. This
allows damping of the extra LC section without high dissipation in
the damping resistor due to DC currents through the damping
resistor.
[0016] Thus, the invention is based on two notions. One is the
insight that the DC power dissipation in the damping resistor can
be avoided, either by putting the damping resistor in series with a
capacitor, thus blocking DC current, or by putting the damping
resistor in parallel to an inductor, thus providing a DC current
bypass because the resistance of the inductor is lower than that of
the resistor. The other insight is that, in order to improve the HF
(High Frequency) suppression of the filter, the HF behaviour should
not be governed by the damping resistor, but must be governed by
second-order LC behaviour.
[0017] The series arrangement of the capacitor and the damping
resistor which conducts negligible DC current can be obtained by
two equivalent circuits. In the first circuit, a capacitor is
arranged in series with the damping resistor, and this series
arrangement is arranged in parallel with the first capacitor which
is arranged in the main current path between the first inductor and
the reference voltage level. In a second circuit, a capacitor is
arranged in parallel with the damping resistor, and the parallel
arrangement is arranged in series with the first capacitor.
[0018] The DC current through the resistor in parallel with the
extra inductor is relatively small because the resistance of the
resistor is relatively large with respect to the resistance of the
inductor with which the series arrangement is arranged in parallel.
This parallel arrangement can be obtained by two equivalent
circuits. In the first circuit, the inductor is arranged in series
with the damping resistor, and the series arrangement is arranged
in parallel with the second inductor which is arranged in the main
current path between the first inductor and the load. In a second
circuit, the inductor is arranged in parallel with the damping
resistor, and the parallel arrangement is arranged in series with
the second inductor. A same reasoning holds for low frequency
currents. On the other hand the HF suppression of the filter is
optimal because it is not degraded to a first order filter.
[0019] In an embodiment as claimed in claim 2, the second current
provides the DC and low frequency portion of the load current, and
the first current provides the high frequency portion of the load
current. A crossover frequency is defined as the frequency at which
the magnitude of the high frequency contribution is equal to the
magnitude of the DC and low frequency contribution. The bandwidth
of the low-pass filter is selected above the crossover frequency
such that its current transfer magnitude is sufficiently large at
the crossover frequency and the filter does not jeopardize the
control loop stability.
[0020] In an embodiment as claimed in claim 3, the bandwidth of the
low-pass filter is selected below a switching frequency of the
DC-DC converter to obtain a current transfer suppression of the
filter at the switching frequency.
[0021] In an embodiment as claimed in claim 4, the low pass filter
comprises the second inductor and the series arrangement of the
second capacitor and the damping resistor. The second capacitor has
an impedance which is at least two times smaller than the impedance
of the first capacitor. To effectively influence the filter
performance, the impedance of the second capacitor should be at
least two times, but preferably at least ten times, smaller than
the impedance of the first capacitor.
[0022] In an embodiment as claimed in claim 5, the first capacitor,
the second capacitor and the second inductor form a resonance
circuit which has a first resonance frequency determined by values
of the first capacitor, the second capacitor and the second
inductor, and a second resonance frequency determined by the first
capacitor and the second inductor. The first resonance frequency is
lower than the second resonance frequency. The values of the first
capacitor, the second capacitor and the second inductor are
selected to obtain a second resonance frequency which is lower than
a switching frequency of the DC-DC converter and which is higher
than a crossover frequency. The crossover frequency is defined as
the frequency at which the magnitude of the first current, which
contains the high frequency portion of a total current through the
load, is equal to the magnitude of the second current, which
contains a DC and low frequency portion of the total current
through the load.
[0023] In an embodiment as claimed in claim 6, the low pass filter
comprises the second inductor and the series arrangement of the
third inductor and the damping resistor. To effectively influence
the filter performance, the third inductor has an impedance which
is at least two times, but preferably at least ten times, smaller
than the impedance of the second inductor.
[0024] In an embodiment as claimed in claim 7, the first capacitor,
the second inductor, and the third inductor form a resonance
circuit which has a first resonance frequency determined by values
of the first capacitor and the second inductor, and a second
resonance frequency determined by the first capacitor, the second
inductor, and the third inductor. The first resonance frequency is
lower than the second resonance frequency. The values of the first
capacitor, the second inductor, and the third inductor are selected
to obtain a second resonance frequency which is lower than a
switching frequency of the DC-DC converter and higher than the
crossover frequency. Again, the crossover frequency is defined as
the frequency at which the magnitude of the first current
containing the high frequency portion of the total current through
the load, is equal to the magnitude of the second current
containing the DC and low frequency portion of the total current
through the load.
[0025] In an embodiment as claimed in claim 8, the linear amplifier
comprises a first amplifier stage, a second amplifier stage, and a
differential input stage. The differential input stage has a
non-inverting input to receive a reference signal, an inverting
input to receive a voltage proportional to a system output voltage
across the load, and an output coupled to both an input of the
first amplifier stage and an input of the second amplifier
stage.
[0026] The first amplifier stage has an output directly connected
to the load to supply the first current to the load. By directly
connecting the output of the first amplifier stage to the load, the
sense resistor in series with the output of the first amplifier
stage, which usually is present to obtain a control voltage for the
DC-DC converter, is not required. The first amplifier stage and the
second amplifier stage have matched components to obtain a third
current which is proportional to the first current. The DC-DC
converter comprises a controller which has a control input to
receive a voltage generated by the third current to control the
second current, which is supplied by the DC-DC converter to the
load, such that the DC-component of the first current is
minimized.
[0027] These and other aspects of the invention are apparent from
and will be elucidated with reference to the embodiments described
hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] In the drawings:
[0029] FIG. 1 shows a block diagram of an apparatus comprising the
power supply system in accordance with the invention,
[0030] FIG. 2 shows a block diagram of a power supply system and a
circuit diagram of an embodiment of the low-pass filter,
[0031] FIG. 3 shows a block diagram of a power supply system and a
circuit diagram of another embodiment of the low-pass filter,
[0032] FIG. 4 shows a circuit diagram of yet another embodiment of
the low-pass filter, and
[0033] FIG. 5 shows a circuit diagram of yet another embodiment of
the low-pass filter.
[0034] It should be noted that items which have the same reference
numbers in different Figures, have the same structural features and
the same functions, or are the same signals. Where the function
and/or structure of such an item have been explained, there is no
necessity for repeated explanation thereof in the detailed
description.
DETAILED DESCRIPTION
[0035] FIG. 1 shows a block diagram of an apparatus which comprises
the power supply system in accordance with the invention. By way of
example only, the apparatus shown is a telecom system. The power
supply system is advantageous in any other apparatus which requires
an efficient and fast power supply which is able to change the
output voltage at a fast speed, or which is able to respond quickly
to a change in the load of a circuit of the apparatus.
[0036] A power efficient RF (high frequency) power amplifier RA for
use in, for example, 2.5G, 3G, or 4G telecom systems requires a
fast and power efficient supply modulator. This supply modulator or
power supply system supplies a rapidly varying supply voltage VO to
the RF power amplifier RA. The supply voltage VO fits the output
power to be supplied by the RF power amplifier RA. A fast and
accurate control of the supply voltage VO, and thus of the current
supplied by the power supply system, is especially important in
handheld battery operated communication devices, such as, for
example, mobile phones, to maximize the time a single battery
charge can supply power to the system. The level of the supply
voltage VO is only high during periods in time wherein a high
output power is required. Thus, as soon as a lower output power is
possible, the level of the supply voltage VO should be rapidly
decreased to optimally fit the lower output power, and the other
way around.
[0037] The power supply system comprises a linear amplifier LA and
a DC-DC converter CO. The linear amplifier LA comprises the
differential input stage OS3 and the amplifier stages OS1 and OS2.
The differential input stage OS3 has an inverting input to receive
a voltage proportional to the output voltage VO, a non-inverting
input to receive the reference voltage VR, and an output to supply
the error signal VE. The amplifier stage OS1 has an input to
receive the error signal VE and an output to supply the output
current I1 of the linear amplifier LA directly to the load which
now comprises the RF power amplifier RA. The amplifier stage OS2
has an input to receive the error voltage VE, a differential output
pair to obtain a current I3 through a resistor R3 arranged between
the differential output pair. The current I3 causes a voltage V3
across the resistor R3. The controller (not shown) of the DC-DC
converter CO uses the voltage V3 to control the switches of the
DC-DC converter to obtain the output current I2 of the DC-DC
converter CO. The DC-DC converter comprises a switching part SM and
a low-pass filter FI. The switching part SM comprises the
controller, at switch which is controlled by the controller, and an
inductor which is coupled to the switch to obtain a varying current
in the inductor. The exact topology depends on the type of DC-DC
controller used.
[0038] The current I2' which is supplied by the switching part SM
is filtered by the low pass filter FI to obtain the filtered
current I2 which is supplied to the load. The filter FI suppresses
the ripple of the DC-DC converter CO. The present invention is
directed to the construction of the low-pass filter FI.
[0039] Another reference voltage VR' is fed to the RF power
amplifier RA. Usually the reference voltage VR only comprises
amplitude information while the reference voltage VR' comprises
phase information and may comprise amplitude information. Thus, if
output power of the RF amplifier has to rapidly increase, the
control signal VR commands the power supply system to increase the
currents I1 and I2. The relatively slow DC-DC converter CO cannot
immediately follow a fast step of the reference signal VR. The
difference between the required current to the load and the current
I2 supplied by the DC-DC converter CO will be supplied as the
current I1 by the linear amplifier. Once a stable situation is
reached, the DC and low frequency part of the current required by
the RF power amplifier RA is delivered by the DC-DC converter CO,
and the current I1 adds the high frequency part of the current
required by the RF power amplifier RA and subtracts (part of) the
inherent ripple of the DC-DC converter CO. Instead of the resistor
R3 which converts the current I3 into a control voltage for the
DC-DC converter CO, a capacitor may be used which replaces the
resistor R3, or which is arranged as a Miller capacitor between an
input and an output of an inverting amplifier OS2.
[0040] Instead of the shown topology to control the DC-DC converter
CO which topology comprises the linear amplifier LA which has the
amplifier stage OS1 of which the output is directly connected to
the load, and an amplifier stage OS2 which generates a current I3
proportional to the current I1, alternatively, other topologies may
be used to control the DC-DC converter CO. For example, although
the direct connection of the output of the amplifier OS1 to the
load has the advantage that it is not required adding an element
which senses the current I1, such an element may be present in the
main current loop. This element may be a resistor or another
current sensor. Now, the voltage across the resistor is used to
control the DC-DC converter CO and the amplifier OS2 is not
required anymore. However such a current sensor which is present in
the main current loop of the linear amplifier LA influences the
loop stability and causes a relatively high dissipation.
[0041] FIG. 2 shows a block diagram of a power supply system and a
circuit diagram of an embodiment of the low-pass filter.
[0042] The switching part SM of the DC-DC converter CO comprises a
controller CON, a switch SC, a switch SY, and an inductance L1. The
switches SC and SY have main current paths which are arranged in
series to receive an input supply voltage VI. One end of the
inductance L1 is connected to the junction of the main current
paths of the switches SC and SY. The controller controls the
switches SC and SY with control signals DR1 and DR2, respectively.
It has to be noted that the switching part SM shown is an example
only. The inductance L1 may be a coil or a transformer. The present
low-pass filter FI can also be advantageously used together with
other DC-DC converters.
[0043] The linear amplifier LA comprises an inverting input to
receive a voltage VO' proportional to the output voltage VO, a
non-inverting input to receive the reference voltage VR, an output
to supply the output current I1 directly to the load LO, and an
output to supply the current I3 to the controller CON of the
switching part SM of the DC-DC converter CO. The current I3 may be
converted to a voltage before being fed to the controller CON. The
linear amplifier LA may be constructed identical to what is shown
in FIG. 1. The controller CON receives the current I3 to control
the switches SC and SY to obtain a current I2 such that the average
value of the current I1 is substantially zero.
[0044] The low-pass filter FI is arranged between the free end of
the inductance L1 at a node NA and the load LO at a node NB. The
load LO comprises a parallel arrangement of a smoothing capacitor
CL and the load impedance RL which often is a resistance. The
current through the load LO is referred to as IT. The low-pass
filter FI comprises an inductor L2 which is arranged between the
nodes NA and NB, a capacitor C1 arranged between the node NA and
ground, and a series arrangement of the capacitor C2 and the
resistor R2 arranged between the node NA and ground.
[0045] In the now following, the dimensioning of the low-pass
filter FI is elucidated for a practical realization. This is an
example only, other practical implementations are possible as well.
A first important parameter is the switching frequency of the DC-DC
converter CO, which is 10 MHz in this particular example. The DC-DC
converter CO adds a ripple current to the system. The additional
filter FI should suppress this ripple. Another important frequency
is the crossover frequency at which the contribution to the load
current IT of the output current I2 of the low-pass filter FI is
substantially equal in magnitude to the contribution to the load
current IT of the output current I1 of the linear amplifier LA. In
the example discussed, the crossover frequency is 0.2 MHz.
[0046] The additional low-pass filter FI should be designed to
obtain a current transfer magnitude which is sufficiently large at
the crossover frequency. Now, the filter does not jeopardize the
control loop stability. While at the switching frequency its
current transfer suppression is sufficiently large to obtain
sufficient ripple suppression.
[0047] The low-pass filter shown in FIG. 2 has two resonance
frequencies:
FRES 1 = 1 2 .pi. L 2 ( C 1 + C 2 ) ##EQU00001## FRES 2 = 1 2 .pi.
L 2 C 1 ##EQU00001.2##
wherein FRES1<FRES2.
[0048] For small values of the damping resistance R2, the filter
will resonate at frequencies close to the resonance frequency
FRES1, whereas for large values of the resistance R2 it will
resonate at frequencies close to the resonance frequency FRES2.
[0049] In a practical realization of the low-pass filter, the
capacitor C2 must have a value which at least is two times the
value of the capacitor C1, but which preferably is a factor 10 to
100 larger, such that the series arrangement of the capacitor C2
and the resistor R2 effectively influences the filter performance.
The resonance frequency FRES2 must be selected lower than the
switching frequency, and higher than the crossover frequency. For
example, the resonance frequency FRES2 may be selected to be 1.4
MHz. The value of the inductor L2 is determined by parameters such
as the required rate-of-change in time of the filter output current
I2, a volume and size of the inductor L2, and a saturation current
limit of the inductor L2. In the present example wherein the
switching frequency is 10 MHz, preferably, the value of the
inductor L2 is selected within the range from 0.1 .mu.H to 5 .mu.H.
By way of example, the value of the inductor L2 is selected to be 1
.mu.H. The value of the capacitor C1 is then 12 nF. The value of
the capacitor C2 is selected a factor 22.5 larger than the value of
the capacitor C1: C2=270 nF.
[0050] For the damping resistor R2 preferably values are chosen
which are in a range around a characteristic impedance ZKAR2:
ZKAR 2 = L 2 C 1 C 2 ##EQU00002##
Preferably, the range for the resistance value of R2 is defined by
values between a lower limit which is 5 times smaller than
characteristic impedance ZKAR2 and an upper limit which is 5 times
larger than characteristic impedance ZKAR2. In the example
discussed, the characteristic impedance ZKAR2=4.2.OMEGA., and the
resistance value of R2 may be selected from the range 1 to
20.OMEGA., for example: R2=4.7.OMEGA..
[0051] In another embodiment in accordance with the invention, an
inductor is added to the series arrangement of the capacitor C2 and
the damping resistor R2, such that the series arrangement of the
inductor, the capacitor C2 and the resistor R2 is arranged in
parallel with the capacitor C1. Again the impedance of the
capacitor C2 is smaller than the impedance of the capacitor C1. The
series circuit of the inductor, the capacitor C2 and the resistor
R2 may be tuned to the switching frequency, or to another frequency
substantially above the -3 dB bandwidth of this low-pass
filter.
[0052] FIG. 3 shows a block diagram of a power supply system and a
circuit diagram of another embodiment of the low-pass filter. This
power supply system is based on the one shown in FIG. 2. The only
difference is that the series arrangement of the capacitor C2 and
the resistor R2 is replaced by a series arrangement of the inductor
L3 and the resistor R3. The latter mentioned series arrangement is
arranged in parallel with the inductor L2.
[0053] Again two resonance frequencies can be indicated:
FRES 1 = 1 2 .pi. L 2 C 1 ##EQU00003## FRES 2 = 1 2 .pi. L 2 L 3 L
2 + L 3 C 1 ##EQU00003.2##
wherein FRES1<FRES2.
[0054] For large values of the damping resistor R3 the filter
resonates at frequencies close to the resonance frequency FRES1,
whereas for small values of the damping resistor R3 it resonates at
frequencies close to the resonance frequency FRES2.
[0055] In a practical realization of the low-pass filter, the
inductor L3 must have a value which at least is two times smaller
than the value of the inductor L2, but which preferably is a factor
10 to 100 smaller, such that the series arrangement of the inductor
L3 and the resistor R3 effectively influences the filter
performance. The resonance frequency FRES2 must be selected lower
than the switching frequency of the DC-DC converter, and higher
than the crossover frequency. The value of the inductor L2 is
determined by parameters such as the required rate-of-change in
time of the filter output current I2, a volume and size of the
inductor L2, and a saturation current limit of the inductor L2. In
the present example wherein the switching frequency is 10 MHz, the
value of the inductor L2 is preferably selected out of the range
from 0.1 .mu.H to 5 .mu.H.
[0056] For the damping resistor R3 preferably values are chosen
which are in a range around a characteristic impedance ZKAR3:
ZKAR 3 = L 2 L 3 C 1 ##EQU00004##
Preferably, the range for the resistance value of R3 is defined by
values between a lower limit which is 5 times smaller than
characteristic impedance ZKAR3 and an upper limit which is 5 times
larger than characteristic impedance ZKAR3.
[0057] In a practical embodiment the following values are selected:
the resonant frequency FRES2 is 1.4 MHz, the inductor L2=1 .mu.H,
the inductor L3=100 nH, the capacitor C1=150 nF, the characteristic
impedance ZKAR3=1.5.OMEGA., and the resistor R3 is selected within
the range from 0.3 to 10.OMEGA.. For example, the resistor
R3=1.5.OMEGA..
[0058] It has to be noted that it is known that a LC filter can be
damped by adding a damping resistor. However, as these filters are
usually implemented in applications in which small currents are
flowing, the dissipation in the damping resistor is not an issue.
These known damping solutions are in the now following discussed
with respect to the embodiments in accordance with the invention as
shown in FIGS. 2 and 3.
[0059] In one prior art solution, the capacitor C2 in FIG. 2 is not
present. Or analogously, in FIG. 3, the series arrangement of the
resistor R3 and the inductor L3 is not present, and the damping
resistor R3 is arranged in series with the inductor L2. This
approach has the advantage that a good high-frequency suppression
is obtained but has the drawback that a high DC power dissipation
occurs in the resistor.
[0060] In another prior art damping technique the capacitor C1
shown in FIG. 2 or the inductor L3 in FIG. 3 is not present.
Although these techniques do not suffer from the additional DC
power dissipation, they have the drawback of reduced high-frequency
suppression with respect to the fourth order two LC-section filter
disclosed in U.S. Pat. No. 5,905,407. In the FIGS. 2 and 3, which
are amended as discussed above, for high frequencies, the
second-order section with capacitor C2 and inductor L2 behaves as a
first-order section with resistor R2 and inductor L2, and as a
first-order section with capacitor C2 and resistor R3,
respectively. Thus, instead of a fourth order filter, only a third
order filter is obtained.
[0061] The invention has the objective to avoid additional DC power
dissipation in the damping resistor, while providing good HF
suppression (namely fourth-order LC behaviour).
[0062] In another embodiment in accordance with the invention, a
capacitor is added parallel to damping resistor R3, such that the
parallel arrangement of resistor R3 and the capacitor is arranged
in series with inductor L3. Again the impedance of the inductor L3
is smaller than the impedance of the inductor L2. The series
circuit of the inductor L3 and the parallel arrangement of the
capacitor and the resistor R3 may be tuned to the switching
frequency, or to another frequency substantially above the -3 dB
bandwidth of this low-pass filter.
[0063] FIG. 4 shows a circuit diagram of yet another embodiment of
the low-pass filter. FIG. 4 shows the part of FIG. 2 including the
first inductor L1 and the low-pass filter FI which is arranged
between the nodes NA and NB. The parallel arrangement of the
capacitor C1 with series arrangement of the capacitor C2 and the
damping resistor R2 of FIG. 2 is replaced by the equivalent circuit
of the series arrangement of the capacitors CA and CB, and the
damping resistor RB which is arranged in parallel with the
capacitor CB. The series arrangement of the capacitors CA and CB is
arranged between the node NA and the reference voltage level (GND).
The capacitor CA replaces the capacitor C1 of FIG. 2.
[0064] The values of the capacitors CA, CB and the resistor RB can
be easily determined from the values selected for the equivalent
circuit shown in FIG. 2:
CA=C1+C2
CB=(C1+C2)*C1/C2
RB=R2*(C2*C2((C1+C2)*(C1+C2)))
[0065] FIG. 5 shows a circuit diagram of yet another embodiment of
the low-pass filter. FIG. 5 shows the part of FIG. 3 including the
first inductor L1 and the low-pass filter FI which is arranged
between the nodes NA and NB. The series arrangement of the damping
resistor R3 and the inductance L3 is replaced by a parallel
arrangement of the inductance LD and the damping resistor RD. This
parallel arrangement is arranged in series with the inductor LC
which replaces the inductor L2 of FIG. 3.
[0066] The values of the inductors LC, LD and the resistor RD can
be easily determined from the values selected for the equivalent
circuit shown in FIG. 3:
LC=L2*L3/(L2+L3)
LD=L2*L2/(L2+L3)
RD=R3*(L2*L2/((L2+L3)*(L2+L3)))
[0067] It should be noted that the above-mentioned embodiments
illustrate rather than limit the invention, and that those skilled
in the art will be able to design many alternative embodiments
without departing from the scope of the appended claims.
[0068] In the claims, any reference signs placed between
parentheses shall not be construed as limiting the claim. Use of
the verb "comprise" and its conjugations does not exclude the
presence of elements or steps other than those stated in a claim.
The article "a" or "an" preceding an element does not exclude the
presence of a plurality of such elements. The invention may be
implemented by means of hardware comprising several distinct
elements, and by means of a suitably programmed computer. In the
device claim enumerating several means, several of these means may
be embodied by one and the same item of hardware. The mere fact
that certain measures are recited in mutually different dependent
claims does not indicate that a combination of these measures
cannot be used to advantage.
* * * * *